1. Field
A method of removing a BARC (buried anti-reflective coating or back anti-reflective coating) from an underlying surface without damaging the underlying surface.
2. Description of the Background Art
This section describes background subject matter related to the disclosed embodiments of the present invention. There is no intention, either express or implied, that the background art discussed in this section legally constitutes prior art.
A BARC (buried ARC or back ARC) refers to an anti-reflective coating which is typically used during a photolithography process of the kind commonly used to pattern layers in a semiconductor device. Often, the BARC is a “hard” layer of anti-reflective material which is applied directly over the surface of a layer which is to be patterned. Overlying the BARC is a photoresist which is processed to provide a pattern in the photoresist layer. A plasma etching process is typically used to transfer the pattern from the photoresist layer through the BARC layer, and to other layers underlying the BARC layer.
The photoresist is patterned using imaging radiation, and the BARC reduces radiation reflection back from surfaces underlying the BARC (the reflected radiation deteriorates the pattern image in the photoresist).
The photoresist is typically a “soft” organic material. The BARC may be an inorganic material, but may also be a “hard”, high molecular weight organic material in some instances. Often the BARC has chemical structure which not only stops the backward reflection of radiation, but also provides functionality as a hard masking material during transfer of the pattern to surfaces underlying the BARC. In many instances, the surface directly underlying the BARC is a low k dielectric material.
Many of the better low k dielectric materials, having a dielectric constant of about 2.9 or lower, are porous in nature, as the porosity contributes substantially to a reduction in dielectric constant of the dielectric material. The porosity which reduces the dielectric constant also causes the dielectric material to be fragile. The techniques used to remove residual BARC from the low k dielectric materials have damaged the surface of the fragile low k dielectric material. Not only is the structure weakened, but the dielectric constant of the low k dielectric material is often increased by the BARC removal process.
More recent efforts to provide an improved BARC removal process have involved methods of protecting the low k dielectric surface prior to BARC removal and then de-protecting the low k dielectric surface subsequent to BARC removal. These methods require additional steps and result in time loss and increased cost in fabrication of the semiconductor device.
To perform the dual function of anti-reflective coating and hard mask, the chemical composition of a BARC layer is substantially different from that of a photoresist layer, and the techniques used to remove a photoresist layer (oxygen plasma, sulfuric acid-hydrogen peroxide mixture, organic solvents, ozone water, amine-based and hydroxide-based solutions, for example) are either not effective in removing a typical BARC layer or would cause damage to commonly used low k dielectric layers underlying the BARC layer.
The low k dielectric layers are frequently used in contact with a surface of a diffusion barrier layer which prevents migration of conductive materials into adjacent semiconductor or dielectric layers. It is important that there be good adhesion between the low k dielectric layer and the diffusion barrier layer. This is important for device performance integrity. It is also important during fabrication of the devices, because the most commonly used processes for creating “multi-layer metal” devices (i.e. multi-layered connectivity devices which are useful in reducing device size) are damascene processes which make use of flattening, milling processes such as chemical mechanical polishing. These milling processes create stress at interfaces between layers present in the device structure at the time of milling.
The stresses created between device layers during a process such as chemical mechanical polishing (CMP) can deform the device, separate interfacial surfaces, and cause performance defects.
A sampling of low k dielectric materials which have been developed in recent years are described below, for example. This is not an all inclusive list of the background art, but hopefully provides a general understanding of the technology which is improved upon by the present invention.
Schmitt et al. U.S. Pat. No. 7,611,996 B2 describes the deposition of low k dielectric layers using chemical vapor deposition (CVD), and preferably plasma enhanced CVD (PECVD), so that reactant gases used to produce depositing films can be excited by the plasma and a lower temperature is required for film deposition. Another factor affecting the selection of materials to be used for low k dielectric layers is parasitic capacitance between metal interconnects on the same or adjacent layers in a circuit. Parasitic capacitance can result in crosstalk between the metal lines or interconnects and/or resistance-capacitance (RC) delay, thereby reducing the response time of the device and degrading the overall performance of the device. To reduce the parasitic capacitance between metal interconnects on the same or adjacent layers, it has been necessary to change the low k dielectric material used between the metal lines or interconnects to a material having an increasingly lower dielectric constant. A dielectric constant below 2.5 is mentioned as desirable in the Schmitt et al. patent. The material developed to obtain this dielectric constant was a nano-porous silicon oxide film having dispersed microscopic gas voids.
U.S. Pat. No. 7,674,755 to Egbe et al, issued Mar. 9, 2010, describes a formulation for removing photoresist, ion implanted photoresist, etch residue or BARC. The formulation comprises an ammonium hydroxide and a 2-aminobenzothiazole, with the remainder being water. (Abstract) U.S. Pat. No. 7,700,533 to Egbe et al., issued Apr. 20, 2010, describes an aqueous cleaning composition used to remove silicon-containing BARC and/or photoresist residue. The composition comprises from about 0.01% to about 40% by weight of a salt selected from Guanidinium salt, an acetamidinium salt, a formamidinium salt, and mixtures thereof; water; and optionally a water soluble organic solvent. (Abstract)
U.S. Pat. No. 7,879,782 to Wu et al., issued Sep. 7, 2010, describes an aqueous-based composition for removing residues such as post-ashed and/or post-etched photoresist from a substrate. The composition includes water; at least one selected from a hydroxylamine, a hydroxylamine salt compound, and mixtures thereof; and a corrosion inhibitor wherein the composition is substantially free of an added organic solvent, and where the corrosion inhibitor does not contain a water soluble organic acid. (Abstract).
In an Advanced Materials article related to removal of hard masking material from low k dielectric surfaces, Adv. Mater. 2011, 23, 2828-2832, Theo Frot et al. describe a strategy of introducing protecting groups to protect a porous low k dielectric material surface during key patterning and processing steps, and then deprotecting the low k dielectric material (regenerating the porosity on the surface of the low k dielectric) prior to subsequent device fabrication steps. While this protection-deprotection scheme may be necessary in some instances, it required additional steps which raise the cost of device protection.
The highly porous low k dielectric materials described above are highly susceptible to damage by the kinds of chemical treatments which have been used to remove photoresists and hardmasks in the past, and there is a need for a method of removing BARC materials which requires a minimal number of steps and which will not damage the underlying low k dielectric materials. The present method enables the removal of residual BARC from an underlying layer of low k dielectric material with minimal harm to the dielectric material and at substantial time and cost savings.